This invention relates to leveling irregular solder mounds on printed circuit boards and more particularly to an air dispersion platen apparatus used in a process for continuous manufacture and leveling of irregularly shaped solder contacts on electronic circuit board panels.
Typically, electronic circuit board panels are produced with multiple solder contacts for mounting multi-lead integrated circuit packages on the circuit board. In producing a printed circuit board panel with solder contacts, a 0.004 inches thick film referred to as a "solder mask" is applied to the panel. The "solder mask" is exposed and developed to produce a cavity at each position of a solder pad. Commonly, a solder paste is then squeegeed onto the panel to fill the cavities. Then, the circuit board panel is heated to cause the solder paste to fuse into solid solder. This fusing process causes the solder to ball up into irregular mounds of solder.
As long as the solder mounds remain irregular, multi-lead integrated circuit packages cannot be effectively soldered to the circuit board panel. For the integrated circuit packages to effectively solder to the circuit board using known soldering processes, the solder contacts must be flat. Flat solder contacts permit the leads on integrated circuit packages to rest in contact with the solder contacts of the printed circuit board. Therefore, when the integrated circuit package is placed on the circuit board and soldered using any one of several known techniques, the package and the circuit board will have a solid solder bond. However, producing circuit board panels with flat solder contacts has been inherently problematic because of the solder contacts proximity to each other and their microscopic size.
Conventionally, the process of flattening and leveling irregular solder mounds on printed circuit board panels has been accomplished by heating the solder until it is molten and then applying direct mechanical pressure by way of platens to Teflon coated belts that press against the circuit board panels. Usually, the belts pass between a stationary platen and a spring loaded platen. Wherein, the platens physically press against the belts toward the printed circuit, pushing the molten solder into the cavities of the solder pads as the solder cools. By the conventional leveling process, the mechanical pressure required to effect proper flattening of the solder contacts is 200 to 400 pounds of total force. Applying this amount of force causes excess friction between the belts and the platens, making it difficult to pull the belts thru the platens. The result is increased heat from friction and increased wear on the belts, shortening the belts' durabilities.
Another problem with the conventional system is that the platens are flat, but the typical circuit board panel is not. A circuit board panel can vary in thickness from side to side and end to end by as much as 0.010 inches. The force of the direct mechanical pressure applied by the platens is concentrated at the thicker areas of the circuit board panel. These uneven concentrations of force increase stress on portions of the belts and shorten the lives of the belts.
The uneven pressures on the panel because of its variable thickness can also cause soldering problems. To achieve reliable soldering, all leads must contact or come within 0.001 inches of the solder pad. Consequently, to produce reliable circuit boards using the present process, only narrow boards with less variances in thickness can be processed.
Another apparatus for flattening solder contacts on circuit boards uses self-contained hydraulic bladders. A shuttle type conveyor transports a printed circuit board between the hydraulic bladders, and the bladders are expanded to press against the circuit board toward the printed circuit and level the irregular solder mounds. A primary drawback to this apparatus is that it requires batch processing of boards, rather than continuous processing. Because it interrupts the flow of work, batch processing is an inherently less efficient means of leveling irregular solder contacts in the production of printed circuit board panels than continuous processing.
A method and apparatus for flattening solder bumps using roller machines is disclosed in U.S. Pat. No. 4,752,027 issued to S. J. Gschwend on Jun. 21, 1988. Solder bumps are reflowed and then cleaned to form discrete solder bumps. Using roller machines, the peaks of the solder bumps are flattened. This method, however, is not continuous and, as a result would be time consuming and likely ineffective for printed circuit boards of modern scale.
As to the application of solder to printed wiring boards, a method for applying solder to printed wiring boards is disclosed in U.S. Pat. No. 5,172,853. The method includes producing a printed wiring board with electrically conductive regions, depositing solder paste on the electrically conductive regions, melting the solder deposits to form hump-shaped solder applications on the printed wiring board, leveling the hump-shaped solder applications by applying pressure to the solder applications in the direction of the printed wiring board, and reinforcing the leveling of the hump-shaped solder applications by heating the applied solder. A related process is described above by way of introducing the subject matter of the invention.
In a field of art substantially unrelated to the invention, Stafford Camp disclosed an apparatus for continuously manufacturing particle boards in U.S. Pat. No. 4,025,272, issued May 24, 1977. In the Camp patent, compressed particle board mats are conveyed by a pair of endless belts between a pair of spaced platens. The spaced platens disclosed include a means for distributing fluid under pressure against the belts. The pressure distributing means includes a plurality of individual subcompartments in each platen. Each subcompartment is supplied with a regulator means for independently controlling the pressure of fluid supplied to each subcompartment. The pressure distributed by the platens causes fluid bearings between the platens and the belts and causes the mat to be dimensioned to is final size by controlling the thickness of the mat. The Camp patent does not suggest the use of air distributing platens in producing printed circuit board panels or flattening solder contacts, but teaches the use of platens having conduits and self-contained chambers in continuously manufacturing board from a mixture of board particles and a bonding agent.
Accordingly, one object of the present invention is to provide a process of producing printed circuit board panels having planar solder contacts while obviating one or more of the limitations and disadvantages of the described prior systems;
Another object of the present invention is to provide an apparatus for producing planar solder contacts on electronic circuit board panels that obviates the disadvantages of know methods;
Another object of the present invention is to decrease the amount of stress on belts during the solder leveling process;
Another object of the present invention is to provide a solder leveling apparatus that reduces uneven concentrations of force by accounting for the varying thicknesses of circuit board panels;
Another object of the present invention is to improve solder leveling on wider circuit board panels.
Yet, another object of the present invention is to improve the quality of the solder joints produced between multi-lead integrated circuit packages and printed circuit board panels.
These and other objects of the invention will be apparent to those persons skilled in this art from the following detailed description of a preferred embodiment of the invention.